Non-line-of-sight (NLOS) point-to-point radio communication is attractive for backhauling small-cell radio base stations (RBS) in urban environment where line-of-sight (LOS) communication is not feasible or too expensive. NLOS operation of microwave links primarily utilizes two physical mechanisms, diffraction and reflection, to bypass obstacles. For some high frequency radio links, in particular links operating at 60 GHz and beyond, diffraction does not allow sufficient power budget due to very high loss and therefore reflection must be the main effect utilized. However, in reality reflections are difficult to use since there must be a wall with correct orientation angle that provides a reflection point connecting the two sites.
FIG. 1 depicts two sites in NLOS condition connected by reflection in a wall. The path by point A works fine while the path by point B does not. For a given geometry there exists only a single point satisfying the law of reflection, i.e. output angle relative the normal to the surface equals the input angle. According to the law of reflection, in FIG. 1, only point A constitutes a reflection point between the two antenna sites, whereas point B does not. Note that the law of reflection must be fulfilled in two dimensions when the first antenna site and the second antenna site are at different heights. In addition, the reflection loss must be stable and sufficiently low in the reflection point which implies that no uneven or movable objects should be present in this point, e.g. balconies, signs, or windows that can be opened. In reality this means that suitable reflection points are very difficult to find and even if there exists a wall at roughly the correct area, the wall may not be possible to use either due to wrong angle, too high loss, or movable items in the way. Another problem hampering large scale NLOS deployment is the difficulty of planning an NLOS link. Without knowledge about the obstacles, e.g. building facades, trees and signs, it is very difficult to plan an NLOS link and would require a visit to the area in order to acquire detailed measurements of the surroundings.
An attractive solution to the problems mentioned above, is to mount reflectors at suitable places that can be aligned to fulfil the law of reflection between the desired sites. At high microwave frequencies such reflector can be small, e.g. below a meter in diameter, and be made almost invisible, e.g. painted with appropriate color or even made in glass (armored with metal). This solution makes it possible to engineer an NLOS link with low loss and stable performance still with high system margin. FIG. 2 exemplifies a path assisted by an alignable passive reflector.
In FIG. 2 the LOS path between a first antenna site and second antenna site is blocked by a building. An NLOS path can be created by mounting a passive reflector 220 at a third site that is positioned such that the law of reflection is fulfilled. Hence, a radio link between the first antenna site and the second antenna site is possible. Another solution could of course be to put active repeaters the third site or on top of the blocking building. However, active repeater would require electricity and access rights for servicing and would thus be a much more expensive solution. The use of passive reflectors allows for very low cost installation both from a reliability point of view and space rental cost. Passive reflectors can also be used in free-space optical (FSO) links to enable NLOS deployment. In such links, the lenses can be very small, e.g. in the diameter range of a few centimeters or decimeters.
The main problem is then how to align the passive reflectors so they accomplish the situation exemplified by point A in FIG. 1. Some suggestions have been to use temporary radio receivers that line up with the reflector in correct directions. This solution is rather complicated and requires the equipment at the first antenna site and the second antenna site to be mounted and almost correctly aligned and must of course be powered on. Thus commissioning such link is difficult. Another solution is to put two temporary radios at the reflection site and align the first part of the link thereafter the second part of the link. When we know that the radios at the two antenna sites are correctly aligned, the reflector is aligned by remotely monitor the received power at one or both of the two antenna sites. Also this installation method is very tedious and requires expensive equipment.
The above methods for alignment of a passive reflector for NLOS radio links are complicated, take long time and require the radio link to be powered on. Thus massive low-cost deployment is not possible. Hence, there is a need for improved method and tools for alignment of a passive reflector for NLOS applications.